Coating compositions and method of treating ferrous surfaces therewith



2,759,862 Patented Aug. 21, 1956 COATING COMPOSITIONS AND METHOD OF WITREATING FERROUS SURFACES IHERE- John A. Henricks, Lakewood, Ohio, assignor to Devex Corporation, Cleveland, Ohio, a corporation of Ohio No Drawing. Application March 11, 1952, Serial No. 274,657

10 Claims. (Cl. 148-624) This invention relates to the coating of ferrous metal for drawing and forming operations. In particular, it relates to a bath for the formation of an adherent mixed ferrous oxalate and sulfide coating on ferrous metals for drawing and forming operations.

Chemical coating baths are generally directed towards coating a particular ferrous metal. That is, one bath coats stainless steel and another bath coats low carbon steel. In metal forming operations, it is thus necessary to change or alter a bath when the composition of the metal being drawn is changed. This entails considerable labor and inconvenience and discourages use of chemical coating baths.

Furthermore, the major portion of ferrous metals prior to the drawing and forming operations are covered with oxide or mill scale. In order to be chemically coated in ordinary coating baths, this mill scale is removed by molten salt baths or in a pickling bath as is Well known in the art.

When a chemical coating bath such as an oxalate bath is employed in steel forming operations, its composition must be carefully watched and controlled. Regular and systematic analysis of the bath is carried out by an analytical chemist andfchemicals added as required. This entails considerable time, labor, and expense. It often means that unskilled labor cannot be employed to operate the bath and it is one of the reasons Why chemical coating baths are not more widely utilized in the steel industry.

One of the principal drawbacks with prior art oxalate coating baths is the fact that they cannot be operated over a period of time without building up concentrations of ions which poison the bath and change the character of the coating formed. Ordinarily, as the bath is utilized on the production line and as various additives and activators are added to the bath, the concentration of certain ions, particularly sodium ions, builds up to a point where the bath is poisoned so that it forms a mixed, powdery sodium oxalate coating or it no longer coats at all. At this point, the bath has to be dumped and a new one made up.

It is, accordingly, one object of this invention to provide a single, workable, chemical coating bath which coats all compositions of ferrous metals.

It is another object of this invention to provide an oxalate coating bath that is activated so as to coat stainless steel surfaces and yet is balanced so as not to exert straight pickling action on low carbon steel.

It is another object of this invention to provide an oxalate coating bath which coats over and so converts mill scale to provide a workable and drawable coating on the metal.

It is another object of this invention to provide an oxalate coating bath which can be dried upon the coated work to provide stepwise lubricant components.

a sulfur containing activator, a water soluble sulfate etching agent, a water soluble borate salt, and a combination of a Wetting agent and a protective colloid will universally coat ferrous metals with a minimum of maintenance, labor, and expense.

In accordance with this invention, a bath is made up of an aqueous solution containing the following percentages of chemical compounds:

Percent Oxalic acid 4-l5 Water soluble sulfate salt 2-5 Boric acid or water soluble borate salt 2-5 Sulfur containing activator which liberates S02 and S in aqueous solution .12.5 Protective colloid .5-3 Wetting agent .01-3 Balance water.

This bath is preferably balanced by a constant stream of gaseous sulfur dioxide which is bubbled through it. The coating formed is a tight adherent greenish-grey combination of ferrous oxalate and ferrous sulfide. The coating bath is maintained at room temperature or slightly above for low carbon steels and at from 150 to 190 F. for stainless steels and other hard-to-coat alloys. A satisfactory coating can be formed on the Work in from 3 to 10 minutes depending upon the particular ferrous metal being coated.

The concentration of the oxalic acid can be within the range of from 4 to 15% and preferably within the range of 4 to 10%. A higher concentration of oxalic is used for the hard-to-coat alloys and for heavier coatings.

The sulfate salt can be any water soluble sulfate salt which gives the sulfate ion in aqueous solution and which does not precipitate the oxalic acid or otherwise poison the bath. Suitable salts are sodium sulfate, sodium acid sulfate, magnesium and ferrous sulfate, any of which aid the oxalic acid attack.

In addition to or in place of all or part of the boric acid,

one or more water soluble borate salts such as borax, manganese borate, ammonium borate and magnesium perborate can be used. This serves as a buffer and stabilizer for the bath, as Well as a stepwise lubricant component. Preferably from 2 to 5 percent of the boric acid or water soluble boric salts are added to the bath. The sulfur containing activators contemplated by this invention are those sulfur compounds which yield sulfur and sulfur dioxide in aqueous solution. Such compounds usually contain oxygen and sulfur with the sulfur having a valence of less than six. Suitable activators are alkali or ammonium thiosulfates, hydrosulfites, tetrathionates, and sulfites. Sulfur monochloride and mixtures of two or more of the various activators is also contemplated Within the terms of this definition. Satisfactory and readily available activators are sodium thiosulfate and sodium sulfite. From around .1 to 2.5 percent of the sulfur-conraining activators are preferably added to the bath.

Examples of a protective colloid which may be used are one or more of starch, dextrine, gelatine, casein, polyvinyl alcohol, methyl cellulose, agar, and the like. Any

protective colloid which does not decompose or putrefy It is still another object of this invention to provide an 7 on standing long periods of time in solution and which helps peptize the colloidal sulfur from sulfur containing activators and thicken the solution can be used. From around .5 up to around 3% of a protective colloid is added to the bath depending upon the effectiveness of the particular colloid and desired thickness of the bath.

The Wetting agent can be any Wetting agent which is stable and does not break down at a low pH. Suitable wetting agents are alkyl-aryl sulfonates, alkyl-aryl ether sulfonates, and the like. Examples of these are Santomerse No. 1 a water soluble, neutral alkyl-aryl sulfonate I is? powder with the alkyl group having at least four carbon atoms), Monsantos Areskap (a butyl diphenyl phenol sulfonate), Aersol OS (isopropyl naphthalene sodium sulfonate), Nacconol NRSF (sodium alkyl aryl sulfonate) and Naccosol A (sodium alkyl naphthalene sulfonate), Rohn & Haas Triton (sulfonated alkyl phenol ether). From .01 up to 3% and preferably around .1 up to .5 or 1% wetting agent generally should be added to the bath.

The wetting agent and protective colloid together stabilize and peptize the colloidal sulfur from the decomposition of the sulfur containing activator and keep it from precipitating out. They also tend to hold the sulfur d1- oxide in solution and to activate and wet the surface of the ferrous metal. They also pick up waste material such.

terially contribute towards the efficiency and activation of" the bath. The decomposition can be represented as follows: Polythionic sulfur containing activator In order to insure a continuous supply of polythionic acids, the bath is preferably stabilized by a. constant stream of sulfur dioxide which is bubbled through it. The sulfur dioxide tends to reverse the above noted reaction by reacting with colloidal sulfur. From .5 up to or or more free sulfur may be added to reverse the above noted reaction. I have found that the addition of free sulfur and sulfur dioxide gas is especially helpful when the metal being coated is a stainless steel or other hard-tocoat alloy. The term hard-to-coat alloys comprehends those stainless alloys high in either molybdenum, nickel, or titanium and which sometimes require .5 to 5% HCl in addition to the sulfur containing activator.

The addition of free sulfur and sulfur dioxide is of advantage because it activates the bath and insures its coating. ability and at the same time does not build up poisonous concentrations of metal ions such as the alkali metal ions. In ordinary baths, sodium salt activators are added to maintain. the bath strength. This builds up the concentration of sodium which forms a yellow powdery so dium iron oxalate in the coating to destroy the adhesion and strength of thecoating bond. It also poisons the bath by making it passive so that it forms only very thincoatings. The use of sulfur dioxide gas avoids the necessity of adding large amounts of sodium containing activators and at the same time maintains an active bath. It thus minimizes build-up of the sodium ion and inhibition of the bath.

Because of the absence of sodium poisoning and because of the sulfur-dioxide activation, this bath coats over scaled ferrous metals with a tight, adherent mixed coating of ferrous oxalate and ferrous sulfide. The iron oxides of the scale are also partially converted to the softer iron sulfides. This phenomena occurs Whether coating scaled low carbon work or coating scaled stainless work. .In many operations, it is thus possible to eliminate the pickling step between hot forming and coating. When, coating over scale,.it is preferable to bubble sulfur dioxide through the bath and add small quantities of free sulfur.

The following examples illustrate this invention:

Balance water.

The bath was heated to 170 F. and pickled 18-8 stainless steel tubes were immersed therein. Within 5 to 10 minutes-,the tubes were coated with an adherent greenishgrey ferrous oxalate-sulfide coating. The tubes were then withdrawn, rinsed, immersed in a soap and borax solution, dried, and drawn. They were drawn to approximately 35% reduction, andfhad a bright finish.

Example 2 Approximate 5% additional wettable sulfur was added to the-bath of. Example 1. and S02 gas was bubbled through from the bottom of the bath. When the bath reached equilibrium, clean 18-8 titanium stainless tubes were immersed therein. In around 5 minutes they were coated with a tight, adherent ferrous oxalate-ferrous sulfide coating. They were then drawn as in Example 1.

Balance water.

Low carbon" steel discs having thin annealed scale were immersed in this bath and were coated within 5 to 10 minutes. They were then removed from the bath, dried, and then thoroughly dusted with a powdered soap. They were then deep drawn with the soap powder lubricant, and produced a superior finish.

The baths of this invention function with a minimum of a supervision. Additional oxalic acid, sulfur-containing' activator, and other compounds are added from time to time as necessary. The bath strength is maintained by testingfor the free acid and total acid by titration. The bath shouldbe maintained at from 12 to 20 points free acid and 15 to 35 points total acid.

.A suitable mixture for making up baths in accordance'with the subject invention is as follows:

This. mixture. is made up with a strength of 10 oz.- 32 oz. per gallon. After this has been mixed and dissolved into. the bath brought to working temperature, 1-20 oz. per gallon of a sulfur containing activator is added. by sprinkling over the bath surface 1-5% sulfur may also be added to t-hismixture as desired.

Example 4 The following is'a suitable mixture:

Percent Oxalic a 65 Sodium sulfa-ta 2O Boric' acid 5 Santomerse No. 1 2 Hydrochloric acid 2 Starch 3 This is activated by the addition of sodium. thiosulfate, and by the- 2% hydrochloric acid added to activate 16 nickel, 25 chromium and 6 molybdenum alloys, inert to the baths in the above examples.

The scaled 16-25-6 alloy bar stock was given two ten minute immersions in the'bath, with intermediate drainage, and. cold water rinsed after coating. The coated bar stock was thengiven alight lime dip, then driedand drawn using a flowed chlorinated paratfin lubricant. The die life with this refractory metal was excellent.

Example Unpickled stainless steel tube stock was coated in the bath of Example 2 by two six minute immersions in the bath which was held at a temperature of 165 and saturated with $02 at that temperature. The coated tubes were cold Water rinsed, and then hot rinsed in a solution of 2 to 20% of sodium meta-silicate, and then given a lubricant coating by immersion in a hot emulsion of 6% wax and 6% sodium tallow soap, after which they were drained, and oven dried. These tubes were given a 28% reduction by drawing over a mandrel, and drew out to a bright dark finish without excessive mandrel wear.

Example 6 Stainless steel tubing that had been annealed in a lithium atmosphere furnace to produce a typical tight black lithium furnace scale were coated by two five minute immersions in the bath of Example 2, after which they were cold rinsed, hot rinsed, and then given a soap and tallow lubricant coating by immersion in a hot 12% solids lubricant tub. After a thorough drying the tubes were drawn over a bar, sent through reeler rolls lubricated by a dilute soap and borax solution, and then drawn through a sinking die to the final dimension. These operations were conducted at the highest speed that the drawing equipment could attain with both excellent tu'be finish and die life.

It is to be understood that in accordance with the provisions of the patent statutes, variations and modifications of the invention described herein may be made without departing from the spirit of the invention.

What I claim is:

1. In a process of treating ferrous surfaces to produce an adherent coating thereon for subsequent drawing and deforming, the step of subjecting the ferrous surface to an aqueous bath at from room temperature to approximately 190 F. containing the following ingredients:

Percent Oxali'c acid 4-15 Sodium acid sulfate 2-5 Sulfur .5-10 Boric acid .5-5 Sodium thiosulfate .'5-2.5 Protective colloid .5-3 Wetting agent stable at low pH values .0l3

Balance essentially water.

for .a period of time long enough to form a substantial adherent, mixed ferrous oxalate and sulfide coating thereon.

2. A bath comprising an aqueous solution containing:

Percent Oxalic acid 4-15 Water soluble sulfate salt 9-5 A compound selected from the group consisting of boric acid and a water soluble borate salt 2-5 A sulfur containing activator which decomposes in aqueous solution to give S02 and S .t12.5 Protective colloid .5-3 Wetting agent stable at low pH values .0l3

Balance essentially water.

3. A bath comprising an aqueous solution containing:

Balance essentially water.

4. A mixture consisting essentially of Percent Oxalic a i 40-70 Sodium acid sulfate 12-25 5 Boric acid 2 7 Protective colloid 3 Wetting agent stable at low pH values 2 Free sulfur 1 to 5 10 5. In a process of drawing and deforming scaled ferrous metals, the steps of subjecting the scaled ferrous surface to a bath at from room temperature to approximately 190 F. for a period of time sufii'cient to convent the scale to a substantial adherent coating of mixed ferrous oxalate and sulfide, said bath containing the following ingredients:

Percent Oxalic acid 4-15 00 Water soluble sulfate salt 2-5 A compound selected from the group comprising :boric acid and a water soluble borate salt 2-5 Protective colloid .5-3

Wetting agent stable at low pH values .5-3

or Free sulfur .5- 10 Balance essentially water.

gaseous sulfur dioxide being bubbled through said bath to form polythionic acids with said free sulfur.

n "6. The process of claim 5 in which the coated metal is removed from contact with the bath with a substantial adherent coating thereon, it is water rinsed, and then is dipped in a hot aqueous solution of 2 to sodium silicate before the application of the lubricant used for subsequent forming operation.

7. In a process of treating ferrous surfaces to produce a substantially mixed ferrous oxalate and sulfide adherent coating thereon for subsequent drawing and deforming, the step of subjecting the ferrous surface to a bath for a time interval suflicient to form a substantial coating and at a reactive temperature within the range of from approximately room temperature to approximately 190 F. made up of an aqueous solution containing the following compounds:

Percent 40 Oxalic acid 445 Water soluble sulfate salt 2-5 A compound selected from the group consisting of boric acid and a Water soluble borate salt 2-5 A sulfur containing activator which decomposes in aqueous solution to give S02 and S .1-2.5 Protective colloid .0l3 Wetting agent stable at low pH values .-5-3 Balance water.

8. The process of claim 7 in which said bath contains .5-10% free sulfur and gaseous sulfur dioxide is bubbled through the bath.

9. In a process of treating ferrous surfaces to produce an adherent coating thereon for subsequent drawing and deforming, the step of subjecting the ferrous surface to an aqueous solution containing the following ingredients:

Balance essentially water.

for a period of time long enough to form substantially adherent, mixed ferrous oxalate and sulfide coating thereon, gaseous S02 being bubbled through said bath.

7 10. The process of claim-'9 in which the ferrous sur- Refergnces C ited in thg file of this patent 'UNITED ST'K'TES' PATENT-s 

1. IN A PROCESS OF TREATING FERROUS SURFACES TO PRODUCE AN ADHERENT COATING THEREON FOR SUBSEQUENT DRAWING AND DEFORMING, THE STEP OF SUBJECTING THE FERROUS SURFACE TO AN AQUEOUS BATH AT FROM ROOM TEMPERATURE TO APPROXIMATELY 190* F. CONTAINING THE FOLLOWING INGREDIENTS: 